This invention relates to the art of heat transfer, and more particularly to a new and improved heat exchanger for use in a refrigerated air dryer.
One area of use of the present invention is in refrigerated air driers wherein warm, moist air such as from the interior of a factory, and which typically is compressed, is cooled and dried and then conveyed to a location where it is used.
In any compressed air system, it is important to reduce the water content of the compressed air as much as possible before delivering the compressed air to the points of use. This is accomplished by using air or water cooled aftercoolers, moisture separators, and air dryers. Air dryers are available in many different types, and the present invention is illustrated with a non-cycling direct expansion refrigerated air dryer wherein the compressor operates continuously. This type of air dryer effectively reduces water content in compressed air by physically chilling the compressed air directly with a refrigeration circuit and thus reducing the capacity of the compressed air to hold water vapor. The water vapor in the chilled compressed air condenses out to liquid droplets. This combination of air and water droplets flows to a moisture separator that mechanically removes the droplets from the air stream. The main components in this type of refrigerated air dryer are the refrigeration system, the moisture separator, and two compressed air heat exchangers.
The first of these heat exchangers is a precooler/reheater. It precools warm saturated compressed air from the air compressor aftercooler by transferring heat to chilled air that is returning from the moisture separator. This part of the process also has the effect of reheating the chilled air before distribution to the end users. The importance of this heat exchanger is that it reduces some of the cooling load that the refrigeration system would otherwise have to handle. The refrigeration system becomes smaller, requiring less power for thriftier operation. The precooler/reheater heat exchanger sometimes is called an "economizer" because of this benefit. Another benefit offered by this heat exchanger is that it reheats the chilled air coming from the moisture separator. Reheating the chilled air reduces the chances that low ambient conditions can cause condensation in the air line downstream of the dryer. Reheating also reduces the likelihood of pipeline condensation or "sweating" that can occur on chilled surfaces in humid conditions.
The second heat exchanger is the air chiller. It takes the precooled air from the "economizer" and chills it down to the desired dewpoint temperature by rejecting heat into evaporating refrigerant on the other side of the heat exchanger. After being chilled, the air enters a moisture separator to remove the condensed water and then the air returns to the "economizer" for reheating.
In conventional refrigerated air dryers the foregoing heat exchangers appear in a variety of types. Most typical are the shell and tube, tube in tube, and plate type heat exchangers. Shell and tube heat exchangers tend to be heavy and they can be costly. Tube in tube heat exchangers are limited to the lower capacity air dryers. The shell and tube type and the tube in tube type heat exchangers both share the disadvantages of not being very compact and they require interconnecting piping between the economizer and the chiller. This adds weight, cost, complexity, and pressure drop to the system. Plate type heat exchangers made from brazing formed plates together may be compact but they lack flexibility due to tooling requirements. Piping to a brazed plate heat exchanger is difficult because the connection locations are so close together that they allow very few options for arranging the connections. Brazed plate heat exchangers generally use copper brazed stainless steel plates in their manufacture. This offers corrosion resistance but it makes them heavy and more costly.
Attempts in the prior art at building combination heat exchangers for air dryers have been counter current designs. Counter current heat exchangers have greater effectiveness but many designs require some kind of intermediate channeling when used in combinations for refrigerated air dryers. The result is more compact than two separate heat exchangers but many of the other disadvantages still remain. Combination heat exchangers made from copper brazed stainless steel plates can be pure counter current designs without any intermediate channeling. However, they still retain many of the disadvantages associated with this type of heat exchanger.